Already defined variable and default(omit)

[danopt]

We need an improvement when we use the default(omit) filter.

The problem is once a variable is defined that could be omitted, default(omit) will not work again. It's not possible to declare the variable as "undefined" again. The filter will not work again, even if the variable is set to null or false.

The solution is simple:
Instead of default(omit) we should always use default(omit, true) or something similar.

In that case, if the variable is set to false or undefined, the parameter will be omitted.